1. Field of the Invention
The present invention relates to a pattern inspecting method for inspecting presence/absence of a pattern defect in a sample, and in particular to a pattern inspecting method used in a pattern inspecting apparatus for inspecting presence/absence of a defect in a fine pattern formed on a transfer mask for lithography used for LSI manufacturing.
2. Related Art
In general, since much cost for LSI manufacturing is required, it is inevitable to improve yield. As one of factors causing reduction in yield, there is a pattern defect in a transfer mask used when a fine pattern is exposed/transferred on a semiconductor wafer by a lithographic technique. In recent years, according to advance of fineness of a dimension of an LSI pattern formed on a semiconductor, the minimum dimension of a defect to be detected becomes fine. Therefore, it is necessary to provide high precision to a pattern inspecting apparatus for inspecting presence/absence of a defect in a transfer mask for lithography used in LSI manufacturing.
Methods for inspecting presence/absence of a defect on a fine pattern of a transfer mask for lithography are roughly classified to Die to Die comparison and Die to Database comparison. The Die to Die comparison is a method for detecting a defect by comparing two dies on a transfer mask, while the Die to Database comparison is a method for detecting a defect by comparing a die and database generated from CAD data for LSI design. The former method has such an advantage that it is unnecessary to prepare database, but it has such a disadvantage that common detect to two dies can not be found. The latter method has such an advantage that there is not any influence due to the common defect, but it has such a disadvantage that it is required to provide a large-scale circuit for generating database.
On the other hand, as a method other than the above methods, there is a comparison method based upon a simultaneous transmission/reflection illumination inspection (see JP-A-08-76359, for example). The comparison method is attached with a trade name “STARlight (Simultaneous Transmission And Reflection light). The method is a method where transmission illumination and reflection illumination are prepared in an optical system and respective images obtained by irradiating the respective illuminations on a transfer mask are compared with each other, which has such a merit that defect detection can be achieved without requesting generation of database even in one chip (two dies which can be compared with each other on a transfer mask are not present). This principle is based upon such a fact that database is complied by using, for example, a neural network to learn correlation between images obtained by transmission illumination and images obtained by reflection illumination in advance in detail, and presence of a defect is determined when the result of comparison of an image of a transfer mask obtained by transmission illumination and an image thereof obtained by reflection illumination deviates from the correlation database.
Incidentally, it is considered that it is difficult to cause the correlation database to function properly when a pixel positional deviation occurs between an image obtained by transmission illumination and an image obtained by reflection illumination or when the both illuminations are different in magnification from each other. Thus, the correlation database includes such a demerit that flexibility is lost in mechanical designing for an inspecting optical system. For example, since a mechanism for an optical system which allows simultaneous acquisition of an image obtained by the transmission illumination and an image obtained by the reflection illumination can not be constructed, the optical system mechanism is limited to such a mechanism that, after an image obtained by transmission illumination is acquired, an image obtained by reflection illumination is acquired. A merit of this method can not be utilized up to the maximum extent regarding its throughput.
Conventionally, as a method for comparing two images of an inspection reference pattern and a pattern to be inspected with each other, such a method is conducted that differences in coordinates between corresponding pixels are calculated and these differences are compared with each other (level comparison), or differences in coordinates between adjacent pixels are calculated and differences between corresponding adjacent pixels are compared with each other (adjacent difference comparison). This approach is called “direct comparison method”.
In recent years, however, it becomes necessary to detect a defect with such a size as buried in a pixel positional deviation, expansion/contraction noise, or sensing noise of an image according to advance of fineness of a pattern on a transfer mask for lithography, which results in limitation in the direct comparison method.